Polymyxins were discovered in 1947 as antibiotics produced by Bacillus polymyxa. Polymyxins are antibiotic decapeptides containing a heptapeptide ring and a N-terminal amide coupled fatty acid. Today, two commercial Polymyxin mixtures are in clinical use; Polymyxin B and Polymyxin E (Colistin). Both mixtures comprise a variety of components as described by Goevaerts et al 2002 and Van den Bossche et al 2011. According to EP pharmacopoeia, Colistin should comprise more than 77% of Polymyxin E1, E2, E3, E1-i and E1-7MOA, but less than 10% of each of the minor components Polymyxin E3, E1-i and E1-MOA.
Due to toxicity associated with Colistin, the mixture was improved by sulfomethylation in the 1950'ties. The sulfomethylated Colistin is called Colistimethate sodium (CMS) which is considered to be a prodrug of Colistin. CMS is still in clinical use as a last-line treatment option for multidrug-resistant organisms such as Pseudomonas aeruginosa, Aqinetobacter baumannii, Klebsiella pneumonia and other Gram negative pathogens. For many years, solutions of CMS have also been administered by nebulization into the lungs of patients with cystic fibrosis (CF) to manage colonization or infections caused by P. aeruginosa. 
The fact that commercial CMS products contain a complex mixture of derivatives of different Polymyxins has several consequences. The foremost of these relates to the therapeutic value of any marketed product. Since CMS may be considered a Colistin-reservoir once injected or inhaled into the body, it is of importance that an appropriate amount of CMS is transformed into colistin before it is excreted. If not, the serum level of Colistin may not reach a level sufficiently high to kill or prevent growth of the pathogenic bacterium targeted. Thus, by being able to manufacture a CMS with a controlled amount of substituents, either in mixture or as mono component, increase therapeutic potential by prevention of under- or over-dosing. It may also increase the prodrug characteristics of the molecule by affecting the hydrolysis rate of the CMS (the in vivo conversion rate of CMS to colistin).